Tag: Polyglutamine Diseases

La huntingtine: un nouvel acteur dans l’arsenal de la réparation de l’ADN

Écrit par Dr. Ambika Tewari, Edité par Dr. Mónica Bañez-Coronel, Traduction française par: L’Association Alatax, Publication initiale: 22 novembre 2019

Des mutations dans la protéine huntingtine altèrent la réparation de l’ADN, causant des dommages importants à l’ADN et une expression génétique modifiée.

Notre génome regroupe l’intégralité de notre matériel génétique, qui contient les instructions pour fabriquer les protéines essentielles à tous les processus de l’organisme. Chaque cellule de notre corps, des cellules de la peau qui constituent une barrière de protection essentielle, des cellules immunitaires qui nous protègent des espèces envahissantes et des cellules du cerveau qui nous permettent de percevoir et de communiquer avec le monde contient du matériel génétique. Au début du développement de chaque espèce de mammifère, il existe une prolifération massive de cellules qui permet le développement d’un embryon au stade une cellule à un corps fonctionnel contenant des trillions de cellules. Pour que ce processus se déroule de manière efficace et fiable, les instructions contenues dans notre matériel génétique doivent être transmises avec précision pendant la division cellulaire et son intégrité maintenue pendant toute la durée de vie de la cellule afin de garantir son bon fonctionnement.

De nombreux obstacles entravent la séquence complexe et hautement orchestrée d’événements au cours du développement et du vieillissement, provoquant des altérations pouvant entraîner un dysfonctionnement cellulaire et une maladie. Les sources de dommages à l’ADN internes et externes bombardent constamment le génome. Les rayonnements ultraviolets et l’exposition à des agents chimiques sont des exemples de sources externes, tandis que les sources internes incluent les processus cellulaires pouvant découler, par exemple, des sous-produits réactifs du métabolisme.

Heureusement, la nature a mis au point un groupe spécial de protéines, appelées protéines de réparation et de réparation de l’ADN, qui permettent aux détecteurs de détecter les messages erronés. Ces protéines spécialisées garantissent que les dommages aux molécules d’ADN qui codent nos informations génétiques ne sont pas transmis à la nouvelle génération de cellules lors de la division cellulaire ou lors de l’expression de nos gènes, protégeant ainsi notre génome. De nombreux troubles génétiques sont causés par des mutations du matériel génétique. Cela conduit à un ARN ou une protéine dysfonctionnel avec peu ou pas de fonction (perte de fonction) ou à un ARN ou une protéine avec une fonction entièrement nouvelle (gain de fonction). Étant donné que les protéines de réparation de l’ADN jouent un rôle crucial dans l’identification et le ciblage des erreurs commises dans le message, il va de soi que toute altération du processus de réparation de l’ADN pourrait conduire à une maladie. Dans cette étude, Rui Gao et ses collègues, par le biais d’une vaste collaboration, ont cherché à comprendre le lien qui existe entre la réparation de l’ADN modifiée et la maladie de Huntington.

 

Un dessin de molécules d'ADN bleues.
Un dessin de molécules d’ADN.

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Snapshot: How does CAG tract length affect ataxia symptom onset?

The instructions our bodies need to grow and function are contained in our genes. These instructions are made up of tiny structures called nucleobases. There are four types of nucleobases in DNA: adenine (A), cytosine (C), guanine (G), thymine (T). By putting these four nucleobases in different orders and patterns, this writes the instructions for our body.

artists drawing of a blue DNA molecule
A cartoon strand of DNA. Image by PublicDomainPictures from Pixabay

Some of the genes contain long sections of repeating ‘CAG” instructions, called CAG tracts. Everyone has repeating CAG tracts in these genes, but once they are over a certain length they can lead to disease. Some ataxias are caused by this type of mutation, including SCA1, SCA2, SCA3, SCA6, SCA7 and SCA17. These are often called polyglutamine expansion disorders. This is because “CAG” gives the body instructions to make the amino acid glutamine. You can read more about what is polyglutamine expansion in our past Snapshot about that subject.

For each disorder caused by a CAG expansion mutation, the number of times the CAG is repeated in a particular gene will determine whether someone will develop the disease. Repeat lengths under this number will not cause symptoms and repeat lengths over the threshold will usually lead to ataxia. When someone undergoes genetic testing for ataxia, doctors will be able to tell them the length of these CAG tracts and whether they have a CAG repeat number in one of these genes that is over the threshold. This table gives a summary of different CAG expansion mutations that can lead to ataxia and how the length of the repeat affects age of onset.

 Affected Gene Normal
Repeat Size 		Disease
Repeat Size
SCA1ATXN16-4439-88
SCA2ATXN215-3136-77
SCA3ATXN312-4055-86
SCA6CACNA1A 4-1821-33
SCA7ATXN74-3537-306
SCA12PPP2R2B4-3266-78
SCA17TBP25-4246-63

For SCA1, SCA2, SCA3, SCA6, and SCA7; longer CAG tracts are associated with earlier onset.

For SCA12, it is hard to predict the age of onset based on repeat length as SCA12 is so rare. Some individuals with long repeats don’t develop ataxia. One study found that longer CAG tract lengths are associated with earlier onset but that it does not affect the severity of symptoms.

For SCA17, Longer CAG tracts have separately been associated with an earlier age of onset and more severe cerebellar atrophy.

In general, people with longer repeat lengths in ataxia genes are likely to present with ataxia symptoms earlier in life. However, it is important to remember that there are many other factors involved. Other genes may have mutations that either worsen the progression of ataxia or protect against more severe symptoms. Therefore, in individual people, the length of the repeat is not always enough information to determine when that person will start showing symptoms, or how severe these symptoms will be.

If you would like more information about the genetic causes of SCAs, including information about genetic testing and what CAG repeat length might mean, take a look at these resources by the National Ataxia Foundation.

Snapshot written by Anna Cook and edited by Larissa Nitschke.

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Huntingtin: a new player in the DNA repair arsenal

Written by Dr. Ambika Tewari Edited by Dr. Mónica Bañez-Coronel

Mutations in the Huntingtin protein impair DNA repair causing significant DNA damage and altered gene expression

Our genome houses the entirety of our genetic material which contains the instructions for making the proteins that are essential for all processes in the body. Each cell within our body, from skin cells that provide a crucial protective barrier, immune cells that protect us from invading species and brain cells that allow us to perceive and communicate with the world contains genetic material. During early development in every mammalian species, there is a massive proliferation of cells that allows the development from a one-cell stage embryo to a functional body containing trillions of cells. For this process to occur efficiently and reliably, the instructions contained in our genetic material need to be precisely transmitted during cell division and its integrity maintained during the cell’s life-span to guarantee its proper functioning.

There are many obstacles that hamper the intricate and highly orchestrated sequence of events during development and aging, causing alterations that can lead to cell dysfunction and disease. Internal and external sources of DNA damage constantly bombard the genome. Examples of external sources include ultraviolet radiation and exposure to chemical agents, while internal sources include cell processes that can arise, for example, from the reactive byproducts of metabolism. Fortunately, nature has evolved a special group of proteins known as DNA damage and repair proteins that act as surveyors to detect erroneous messages. These specialized proteins ensure that damage to the DNA molecules that encode our genetic information is not passed to the new generation of cells during cell division or during the expression of our genes, ultimately protecting our genome. Many genetic disorders are caused by mutations in the genetic material. This leads to a dysfunctional RNA or protein with little or no function (loss of function) or an RNA or protein with an entirely new function (gain of function). Since DNA repair proteins play a crucial role in identifying and targeting mistakes made in the message, it stands to reason that impairment in the DNA repair process might lead to disease. In this study, Rui Gao and colleagues through an extensive collaboration sought to understand the connection between altered DNA repair and Huntington’s disease.

Blue strands of DNA
An artist’s rendering of DNA molecules.

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Snapshot: What is Polyglutamine Expansion?

The information that allows the normal development and functioning of each human being is coded in DNA, which exists in all cells of the body. Several successive segments of DNA make up a gene, with the human body containing approximately 20,000. Every gene has a different arrangement of DNA segments and itself codes for a protein with a specific function. Genes code for proteins in the sequence of their DNA: combination of DNA sequences “code” for different protein precursors called amino acids. Thus, information from DNA (“genes”) codes for amino acids, which come together to form proteins, who function to maintain the normal well-being of the body.

A small number of genes have a small segment of DNA that is repeated successively, usually a couple dozen times, for unknown reasons. When the respective protein is formed, it also possesses a repetition of the same amino acid, corresponding to the repeated DNA segment. These repetitions in proteins have the prefix “poly”, meaning that the amino acids are repeated multiple times in a row, causing an “expansion” in the protein. One of the most common repeated amino acids is called glutamine: hence the name, polyglutamine.

Diagram showing how multiple CAG triplet repeats code for replicates of glutamine to be inserted into a protein
Photo courtesy of NHS HEE Genomics Education Programme.

When there is an increase in the number of repetitions of these segments in DNA, we say that an expansion of the polyglutamine has occurred. When the number of glutamines is increased sufficiently, a disease can develop: we call these disorders “polyglutamine diseases”. Some examples of diseases caused by this polyglutamine expansion are Huntington’s disease, SCA1, SCA2, SCA3, SCA6, and SCA7. The difference between all these diseases is that the expansion of the DNA segment that causes the polyglutamine occurs in different genes. Since these genes are distinct, the way that this expansion interferes with the normal body functioning is also different, giving rise to altered clinical presentations and courses. Moreover, it has been well established that, the larger the number of times that the segment is repeated, the more severe the disease will be. Finally, it has also been observed that throughout each generation, abnormally increased segments tend to become even bigger, making the disease worse.

The discovery of this mechanism of disease has been very important for scientists, since it allows for a “molecular diagnosis” of the disease. Armed with this understanding, research is now focused on understanding this process and finding ways to block the negative effects of polyglutamine expansion.

If you would like to learn more about polyglutamine expansion, take a look at this article.

Snapshot written by Jorge Diogo Da Silva, edited by Dr. Maxime Rousseaux

 

 

DNA Damage Repair: A New SCA Disease Paradigm

Written by Dr. Laura Bowie Edited by Dr. Hayley McLoughlin

Researchers use genetics to find new pathways that impact the onset of polyglutamine disease symptoms

The cells of the human body are complex little machines, specifically evolved to fulfill certain roles. Brain cells, or neurons, act differently from skin cells, which, in turn, act differently from muscle cells. The blueprints for all of these cells are encoded in deoxyribonucleic acid (DNA). To carry out the instructions in these cellular blueprints, the DNA must be made into ribonucleic acid (RNA), which carries the instructions from the DNA to the machinery that makes proteins. Proteins are the primary molecules responsible for the structure, function, and regulation of the body’s organs and tissues. A gene is a unit of DNA that encodes instructions for a heritable characteristic – usually, instructions for a making a particular protein. If there is something wrong at the level of the DNA (known as a mutation) then this can translate to a problem at the level of the protein. This could alter the function of a protein in a detrimental manner – possibly even rendering it totally non-functional.

dna-2358911_1280
Artist representation of a DNA molecule. Image courtesy of gagnonm1993 on Pixabay.

DNA is made up of smaller building blocks called nucleotides. There are four different nucleotides: cytosine (C), adenine (A), guanine (G), and thymine (T). Polyglutamine diseases, such as the spinocerebellar ataxias (SCAs) and Huntington’s disease (HD), are caused by a CAG triplet repeat gene expansion, which leads to the expansion of a polyglutamine tract in the protein product of this gene (MacDonald et al., 1993; Zoghbi & Orr, 2000). Beyond a certain tract length, known as the disease “threshold,” the length of this expansion is inversely correlated with age at disease onset. In other words, the longer this expansion is, the earlier those carrying the mutation will develop disease symptoms. However, scientists have determined that onset age is not entirely due to repeat length, since individuals with the same repeat length can have different age of disease symptom onset (Tezenas du Montcel et al., 2014; Wexler et al., 2004). Therefore, other factors must be involved. These factors could be environmental, genetic, or some combination of both.

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